1. Field of the Invention
This invention relates to the field of object-oriented programming and distributed computing.
2. Background Art
Object-oriented programming is a method of creating computer programs by combining certain fundamental building blocks, and creating relationships among and between the building blocks. The building blocks in object-oriented programming systems are called "objects." An object is a programming unit that groups together a data structure and the operations (methods, or procedures) that can use or affect that data. Thus, an object consists of data and one or more operations or procedures that can be performed on that data. An object can be instructed to perform one of its methods when it receives a "message." A message is a command or instruction to the object to execute a certain method. It consists of a method selection (name of the method) and arguments (values to be assigned to the variables within the object). A message tells the receiving object what to do.
A disadvantage of the prior art object-oriented programming systems is that all objects are required to exist in a single program or process. This prohibits utilizing an object-oriented programming system when writing distributed applications. In addition, these prior art limitations prevent the creation of applications that are distributed physically over networks of machines. One prior art method for providing distributed object-oriented programming is described in "Design of a Distributed Object Manager for the SmallTalk-80 System," D. Decouchant, OOPSLA 86 Proceedings, September, 1986, pp. 444-452. The Decouchant reference describes the design of a distributed object manager that allows several SmallTalk-80 systems to share objects over a local area network. When a local object desires to communicate with a remote object, the local object communicates with a "proxy" that locally represents the remote object. The proxy has two fields that describe a remote object, namely, the resident site of the remote object and a pointer to the object in the resident site. If the referenced object migrates, the contents of the referencing object are not modified. The proxy is updated accordingly by the object manager. In this implementation, a proxy is functionally equivalent to a Unix link, except that a proxy is not visible to the programmer. It is a private data structure which is handled by the object manager like other Small-Talk objects. In U.S. patent application entitled "method for providing automatic and dynamic translation of language-based message passing into operating system message passing," Ser. No. 07/731,636 filed on Jul. 17, 1991, assigned to the assignee of the present application, and hereby incorporated fully be reference into the present application, a method using proxies for overcoming the prior art difficulties with distribution of objects in different processes has been disclosed. As disclosed in that application, a proxy acts as a local receiver for all objects in a local program. When a proxy receives a message, the message is encoded and transmitted between programs as a stream of bytes. The use of proxies, as disclosed in that application, accommodate distributed programming in an object-oriented environment.
In local or distributed object programming environments, objects are constantly creating and disposing of other objects. Much of the time an object creates other objects for its private use and then disposes of them as it needs. However, when for example through invocation of a source object's method, the source object creates and passes an object to a destination object, the lines of ownership -and responsibility for disposal blur. Suppose, for example, that in an Objective C environment an object called Gadget contains a number of objects called Widgets, and that an object called Request wishes to access the Widget objects by invoking the method: -(NSArray *) Widgets. This invocation causes memory space to be allocated to, for example, copies of the Widget objects for use by the Request object. However, this method does not address whether the Gadget object or the Request object should keep track and dispose of the memory space allocated to the Widget objects after the Request object is done with the Widget objects.
A convention can be made that either the source object, i.e. the Gadget object, or the destination object, i.e. the Request object, is responsible for disposing the created object, i.e. the Widget object. Any such convention, however, requires the programmer to keep track of a complex web of objects that are constantly created in response to invocation of various methods. Any such convention also presents other difficulties. For example, in a distributed processing environment the source object may reside in a local machine and the destination object may reside in a remote machine. In that case, the remote machine may run a process that does not have the same programming conventions as the local process. For example, the remote process may run on a different programming language than that of the local process. In such a situation, it is very difficult for a programmer of the local machine to keep track and dispose of objects that are created for the remote machine. One problem is that the local source object needs to know when the destination object is done with the created object since it is clearly inappropriate to dispose of the created object while it is still being used by the destination object. On the other hand, it is difficult to keep track of every destination object of the remote machine and to send messages to the destination objects for disposal of objects that are no longer needed, especially if the programming environments of the remote and local machines are different. In any event, any conventional attempt at resolving the problem of reclaiming memory space involves active programmer involvement in the difficult and complex task of reclaiming memory space allocated to objects created in the course of running a program.
One known attempt in automatic deallocation of the memory assigned to objects that are no longer needed is referred to as "garbage collection."However, it is difficult to provide garbage collection in a machine-independent form in languages (such as C-based languages) in which garbage collection is not built in. In addition, garbage collection is unpredictable and may occur without programmer control since a general garbage collection routine is activated by a single request that can be generated at any instant. In addition, present garbage collection methods might miss certain classes of deallocated objects.
Thus, there is need in the art for an effective and efficient method for reclamation of memory space allocated, for example to objects in an object-oriented programming environment. In particular, there is need for an effective and efficient way for reclaiming memory space in a distributed process environment where objects created in a local machine are passed on to a destination object residing in a remote machine. Moreover, there is a need for a method to transparently, i.e. without actively involving the programmer, reclaim memory space allocated to objects created during the course of running a program.